1. Field of the Invention
The present invention relates to a sensor device for detecting at least one property of a fluid medium flowing in a channel.
2. Description of the Related Art
In many processes, for example in the field of process engineering, chemistry, mechanical engineering or in the field of internal combustion engines, the process must be supplied during certain process steps with a very particular mass of at least one fluid medium having well-defined properties, such as a temperature or a pressure. In particular, combustion processes which are to take place under controlled conditions are dependent on the precise determination of the gas mass and the properties of the fluid used. Different types of sensors are used to determine the flow rate of the fluid medium, its pressure and/or its temperature and are exposed to this fluid medium.
Over the service life of such a sensor device, it is possible for particles present in the flowing fluid to deposit on the sensor element of the sensor device determining the properties of the fluid medium and thus distort the measuring signal over time. In particular, particles which have become electrically charged or electrically polarized during their flow path are known to deposit on the sensor element and then remain permanently adhered thereto due to surface adhesion forces.
An air mass flow meter having a gradient field oil separation is known from published German patent application document DE 10 2005 057 574 A1. This air mass flow meter includes a gradient field generating device in the sensor housing in the area of the sensor element for air mass determination, the gradient field generating device being designed to generate an electrical field which has a field gradient and at least partially permeates the air mass flow. Oil droplets present in the air mass flow are electrically polarized. Due to the gradient field, a net force acts on the oil droplets, which may be used to drive the oil droplets away from the sensor surface and thereby prevent a contamination of the sensor surface.
An air mass flow meter having an electrical oil separation is known from published German patent application document DE 10 2005 057 575 A1. This air mass flow meter includes a field generating device in the sensor housing in the area of the sensor element for air mass determination, the field generating device being designed to generate a preferably at least partially homogeneous electrical field which permeates the air mass flow. In this embodiment of the hot film air mass flow meter, a contamination of the sensor surface by oil droplets may be prevented by electrical effects, in particular a deflection and/or precipitation of coagulated oil droplets.
The approaches described in this way for reducing particle deposits on the sensor element require a more complex design than conventional air-flow sensors and additionally necessitate the application of a high voltage, which is in the range of several 100 V to several 1000 V, to achieve the desired effects. To make such voltages available, either an additional high voltage terminal must be provided at the air-flow sensor or an additional component for voltage conversion is needed. Moreover, these approaches focus only on a removal of electrically charged oil droplets and do not prevent the creation of electrically charged particles.
An air mass flow meter including a housing made of a plastic acting in an electrically insulating manner is known from published German patent application document DE 10 2010 020 264 A1. A flow channel is formed in the housing of the air mass flow meter. In addition, a sensor element, which detects the air mass flowing in the flow channel, is situated in the housing. Furthermore, strip conductors are situated in the housing, which connect the sensor element to connector pins. To discharge electrostatically charged particles in the air mass flow and protect the sensor element in this way from the deposition of these particles, at least a portion of the flow channel situated in the housing has electrically dissipative properties in that this portion of the flow channel is produced, for example, from a plastic including conductive polymers and/or a plastic including conductive fibers and/or from plastic including conductive carbon black.
This approach also does not prevent the creation of electrically charged particles. With this approach, rather, already created electrically charged particles are discharged immediately in front of the sensor element as a result of an impact on the wall of the flow channel situated in the sensor housing. The production of such a device, in which portions of the sensor housing are made of electrically insulating plastic and other portions of the sensor housing are made of electrically conductive plastic, is complex and expensive from a manufacturing perspective.